Charge-transfer and spin-flip states : thriving as complements

Abstract

Transition metal complexes with photoactive chargetransfer excited states are pervasive throughout literature. Especially [Ru(bpy) 3 ] 2+ (bpy = 2,2’-bipyridine) with its long-lived triplet metal-toligand charge-transfer emission has been established as a key model complex in the past decades. Meanwhile, interest in luminescence and reactivity of so-called spin-flip metal-centered states has risen dramatically after the Molecular Ruby [Cr(ddpd) 2 ] 3+ (ddpd = N, N ’dimethyl- N, N ’-dipyridin-2-yl-pyridine-2,6-diamine) unveiled design principles to access strong, long-lived emission from photostable chromium(III) complexes. This review contrasts the properties of emissive charge-transfer and spin-flip states illustrated with the prototypical complexes [Ru(bpy) 3 ] 2+ and [Cr(ddpd) 2 ] 3+ . First, we discuss the nature of the relevant excited states, their tunability with regards to energy and lifetime, and their responses to external stimuli. The final section identifies strengths and weaknesses of the charge transfer and spin-flip states in applications such as photoredox catalysis, sensing and circularly polarized luminescence (CPL).